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(p200300d82f4e3200c99da38b3f3ad4b3.dip0.t-ipconnect.de. [2003:d8:2f4e:3200:c99d:a38b:3f3a:d4b3]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-475c428e9b2sm56333455e9.5.2025.10.22.11.34.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 22 Oct 2025 11:34:56 -0700 (PDT) Message-ID: Date: Wed, 22 Oct 2025 20:34:53 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 7/8] mm: Introduce deferred freeing for kernel page tables To: Lu Baolu , Joerg Roedel , Will Deacon , Robin Murphy , Kevin Tian , Jason Gunthorpe , Jann Horn , Vasant Hegde , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , Alistair Popple , Peter Zijlstra , Uladzislau Rezki , Jean-Philippe Brucker , Andy Lutomirski , Yi Lai , Lorenzo Stoakes , "Liam R . Howlett" , Andrew Morton , Vlastimil Babka , Mike Rapoport , Michal Hocko , Matthew Wilcox , Vinicius Costa Gomes Cc: iommu@lists.linux.dev, security@kernel.org, x86@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Dave Hansen References: <20251022082635.2462433-1-baolu.lu@linux.intel.com> <20251022082635.2462433-8-baolu.lu@linux.intel.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20251022082635.2462433-8-baolu.lu@linux.intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 22.10.25 10:26, Lu Baolu wrote: > From: Dave Hansen > > This introduces a conditional asynchronous mechanism, enabled by > CONFIG_ASYNC_KERNEL_PGTABLE_FREE. When enabled, this mechanism defers the > freeing of pages that are used as page tables for kernel address mappings. > These pages are now queued to a work struct instead of being freed > immediately. > > This deferred freeing allows for batch-freeing of page tables, providing > a safe context for performing a single expensive operation (TLB flush) > for a batch of kernel page tables instead of performing that expensive > operation for each page table. > > Signed-off-by: Dave Hansen > Signed-off-by: Lu Baolu > Reviewed-by: Jason Gunthorpe > Reviewed-by: Kevin Tian > --- > mm/Kconfig | 3 +++ > include/linux/mm.h | 16 +++++++++++++--- > mm/pgtable-generic.c | 37 +++++++++++++++++++++++++++++++++++++ > 3 files changed, 53 insertions(+), 3 deletions(-) > > diff --git a/mm/Kconfig b/mm/Kconfig > index 0e26f4fc8717..a83df9934acd 100644 > --- a/mm/Kconfig > +++ b/mm/Kconfig > @@ -908,6 +908,9 @@ config PAGE_MAPCOUNT > config PGTABLE_HAS_HUGE_LEAVES > def_bool TRANSPARENT_HUGEPAGE || HUGETLB_PAGE > > +config ASYNC_KERNEL_PGTABLE_FREE > + def_bool n > + > # TODO: Allow to be enabled without THP > config ARCH_SUPPORTS_HUGE_PFNMAP > def_bool n > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 52ae551d0eb4..d521abd33164 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -3031,6 +3031,14 @@ static inline void __pagetable_free(struct ptdesc *pt) > __free_pages(page, compound_order(page)); > } > > +#ifdef CONFIG_ASYNC_KERNEL_PGTABLE_FREE > +void pagetable_free_kernel(struct ptdesc *pt); > +#else > +static inline void pagetable_free_kernel(struct ptdesc *pt) > +{ > + __pagetable_free(pt); > +} > +#endif > /** > * pagetable_free - Free pagetables > * @pt: The page table descriptor > @@ -3040,10 +3048,12 @@ static inline void __pagetable_free(struct ptdesc *pt) > */ > static inline void pagetable_free(struct ptdesc *pt) > { > - if (ptdesc_test_kernel(pt)) > + if (ptdesc_test_kernel(pt)) { > ptdesc_clear_kernel(pt); > - > - __pagetable_free(pt); > + pagetable_free_kernel(pt); > + } else { > + __pagetable_free(pt); > + } > } > > #if defined(CONFIG_SPLIT_PTE_PTLOCKS) > diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c > index 567e2d084071..1c7caa8ef164 100644 > --- a/mm/pgtable-generic.c > +++ b/mm/pgtable-generic.c > @@ -406,3 +406,40 @@ pte_t *__pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd, > pte_unmap_unlock(pte, ptl); > goto again; > } > + > +#ifdef CONFIG_ASYNC_KERNEL_PGTABLE_FREE > +static void kernel_pgtable_work_func(struct work_struct *work); > + > +static struct { > + struct list_head list; > + /* protect above ptdesc lists */ > + spinlock_t lock; > + struct work_struct work; > +} kernel_pgtable_work = { > + .list = LIST_HEAD_INIT(kernel_pgtable_work.list), > + .lock = __SPIN_LOCK_UNLOCKED(kernel_pgtable_work.lock), > + .work = __WORK_INITIALIZER(kernel_pgtable_work.work, kernel_pgtable_work_func), > +}; > + > +static void kernel_pgtable_work_func(struct work_struct *work) > +{ > + struct ptdesc *pt, *next; > + LIST_HEAD(page_list); > + > + spin_lock(&kernel_pgtable_work.lock); > + list_splice_tail_init(&kernel_pgtable_work.list, &page_list); > + spin_unlock(&kernel_pgtable_work.lock); > + > + list_for_each_entry_safe(pt, next, &page_list, pt_list) > + __pagetable_free(pt); > +} > + > +void pagetable_free_kernel(struct ptdesc *pt) > +{ > + spin_lock(&kernel_pgtable_work.lock); > + list_add(&pt->pt_list, &kernel_pgtable_work.list); > + spin_unlock(&kernel_pgtable_work.lock); > + > + schedule_work(&kernel_pgtable_work.work); > +} > +#endif Acked-by: David Hildenbrand I was briefly wondering whether the pages can get stuck in there sufficiently long that we would want to wire up the shrinker to say "OOM, hold your horses, we can still free something here". But I'd assume the workqueue will get scheduled in a reasonable timeframe either so this is not a concern? -- Cheers David / dhildenb